Building at Optimization level (fastest ) webgl.

I’m greatly interested in building for webgl with unity3d but the thing is when I try to build it builds well at optimization level (slow) and it hangs my browser (firefox) when i run the index.html file in it. and with optimization level(fastest) it throws errors cant build webgl player.

That sounds like a bug. Please submit one and attach your repo project with it.
http://unity3d.com/unity/qa/bug-reporting